Sorry RJ...just seen your post...have a look at a plan to draw on the wall so as to make your clips square and level...
http://tinyurl.com/y9znejy
Fit the 2 lower fixed clips where the lines cross..
Sit the mirror in the lower clips...
Then slide the spring clips behind the mirror where the vertical lines appear above the mirror...and mark across their top edge.
Remove the mirror and fix the top clips...
Now re-fit the mirror by forcing the top clips up, and then lowering the mirror in on top of the bottom clips...
Hope that makes sense...remember do your lines on masking tape...good luck..!